1. Field of the Invention
The present invention relates to a teleconference terminal connected to other such terminals located at remote points by a network, which can inter-communicatively execute teleconferencing by transmitting audio and video information in a multiplex format using line switching on an ISDN line.
2. Description of the Related Art
In recent years, the use of ISDN lines has become commonplace, and this, as well as the establishment of a CCITT recommendations, such as H series, as standard protocols for teleconference transmission, and the reductions in the cost of TV picture codecs have led to the development of teleconference terminals, based on widely-used personal computers (PC), which are capable of video telephone services using ISDN networks. This kind of teleconference terminal is known as a PC-based teleconference terminal.
FIG. 1 shows a construction of an PC-based teleconference terminal 100 according to the related art. This PC-based teleconference terminal 100 is a terminal for H.320 recommendation on the CCITT, and is principally comprised of a PC 110 and an AV unit 170. The AV unit 170 is constructed as an single enclosed unit, and includes a specialized network control unit 120, a video codec unit 130, an audio codec unit 140, a CPU interface 160, a control bus 161, and a time sharing bus 162. The PC 110, the video codec unit 130, and the audio codec unit 140 are connected to the display unit 150, the camera 151 and the microphone 152 and speaker 153, respectively.
The PC 110 is made up of the CPU 111, a memory 112 and a display control unit 113 and the computer bus 114.
The CPU 111 is composed of a widely-used microprocessor, and executes data processing using the data received from the ISDN line by executing a program stored in the memory 112.
The memory 112 stores the programs to be executed by the CPU 111, as well as providing the working-storage for when the CPU 111 is executing data processing.
The display control unit 113 controls the operation of the display unit 150 and has the video signals expanded and decoded by the video codec unit 130 displayed by the display unit 150. The display control unit 113 is equipped with a function for having a number of images superimposed and displayed by the display unit 150.
The computer bus 114 provides the data transfer lines for connecting all of the components inside the PC 110 with the CPU interface 160 in the AV unit 170.
The specialized network control unit 120 is comprised of a line switching connection processing unit 121, an ISDN line interface unit 122, a network clock control unit 123 and an AV multiplexer/separator unit 124.
The line switching connection processing unit 121, executes the processes connecting and disconnecting the line switching on the D channel of the ISDN line via the ISDN line interface unit 122, in accordance with control data such as call requests from the CPU 111 inputted via the CPU bus 160.
The ISDN line interface unit 122 is a standard ISDN line interface shown at the regulated S/T point.
The network clock control unit 123 generates a clock which is synchronized to the transfer rate of the ISDN line, which it supplies to the line switching connection processing unit 121 and the AV multiplexer/separator unit 124.
The AV multiplexer/separator unit 124 is realized by a specialized LSI circuit, and separates the AV multiframes sent on the B channel of the ISDN line into video code and audio code, which it inputs into the video codec unit 130 and audio codec unit 140, respectively, via the time sharing bus 162. It also converts audio code from the audio codec unit 140 and video code from the video codec unit 130 received via the time sharing bus 162, along with a frame alignment signal (FAS) and a bit-rate allocation signal (BAS), into AV multiframes, which it then transfers to the ISDN line interface unit 122.
The video codec unit 130 is comprised of a D/A convertor 131, an A/D convertor 132, a video clock generation unit 133, and video codec circuit 134. The video codec unit 130 is also connected to the camera 151. The video codec unit 130 performs the compression and encoding of video data and the expansion and decoding of video code, based on the video sampling signal which is synchronized with the video signal from the camera 151. In the following explanation, video information expressed as a digital value which has undergone sampling by the video signal will be referred to as "video data", while compressed and encoded image data will be referred to as "video code".
The D/A convertor 131 converts the video data expanded and decoded by the video codec circuit 134 into an analogue signal and sends it to the display control unit 113.
The A/D convertor 132 converts the analogue signal received from the camera 151 in accordance with the video sampling signal, in doing so generating video data.
The video codec circuit 134 has a memory capacity for one second of frame data, and compresses and encodes the video data received from the A/D convertor 132 in accordance with CCITT recommendation H.261. It also expands and decodes the received video code based on the same standard.
The audio codec unit 140 performs the expansion and decoding of audio data, based on the audio sampling signal which is synchronized with the transfer speed of the ISDN line which is supplied by the network clock control unit 123. Accordingly, the audio sampling signal is a clock signal having a frequency of 8 kHz.
The D/A convertor 141 converts the audio data decoded by the audio codec circuit 143 into an analogue signal, based on the audio sampling signal, and sends it to the speaker 153.
The A/D convertor 142 samples the analogue signal received from the microphone 151 and converts it into a digital signal in accordance with the audio sampling signal, in doing so generating audio data.
The audio codec circuit 143 compresses and encodes the audio data received from the A/D convertor 142 based on TTC standard JT-G722, G728 or suchlike. It also decodes and expands the audio code which is separated from the received AV multiframes, based on a said same standard.
In the following explanation, audio information expressed as a series of digital values which has undergone sampling by the audio signal will be referred to as "audio data", while expanded and decoded audio data will be referred to as "audio code".
The display unit 150, is comprised, for example, of a CRT or an LCD.
The camera 151 is comprised of an image pick-up device such as a CCD (charge couple device).
The microphone 152 converts the inputted sound into an audio signal which is an electric signal, which it then outputs to the A/D convertor 142.
The speaker 153 reproduces the audio information in the audio signal received from the D/A convertor 141.
The CPU interface 160 is connected to the computer bus 114 and the control bus 161, and allows the transfer of control data between the PC 110 and the AV unit 170.
The control bus 161 transmits the control data between the CPU interface 160 and every component in the AV unit 170.
The time sharing bus 162 is connected to the AV multiplexer/separator unit 124, the video codec circuit 134 and the audio codec circuit 143, and transfers video code and audio code using a time sharing method.
In the PC-based teleconference terminal 100 constructed as described above, AV multiframes received on the B channel of the ISDN line are separated by the AV multiplexer/separator unit 124, with the video code and audio code being sent directly to the video codec unit 130 and the audio codec unit 140, respectively, via the time sharing bus 162, without passing through the PC 110. In the same way, video code from the video codec unit 130 and audio code from the audio codec unit 140 are sent directly via the time sharing bus 162, without passing through the computer PC 110, to the AV multiplexer/separator unit 124 where they are converted into multiframes are transmitted on the ISDN line.
However, for the PC-based teleconference terminal 100 under the related art, in order to execute teleconferencing, it has been necessary to provide a general use PC with an AV unit 170 which is separate from the PC 110 and which cannot be directly connected to the computer bus in the PC 110, so that such systems have had the drawbacks of high cost, difficulties regarding reduction in scale, and poor potential for expansion.
More specifically, these problems are as described below.
1. For the PC-based teleconference terminal 100, a specialized network control unit 120 is provided in the AV unit 170, so that if the PC 110 features its own network control unit capable of ISDN line switching, such as an ISDN board, there will be a duplicity of network control units capable of ISDN line switching. Therefore, it can be seen that the hardware resources available in the PC 110 are not being used to their full potential.
2. Since the multiframe conversion and separation are executed by an AV multiplexer/separator unit 124 constructed from a special use LSI circuit which is provided outside the PC 110, the cost of the system becomes high, and reduction of the scale of the system becomes problematic.
3. Since transfer of video code and audio code between the specialized network control unit 120, the video codec unit 130 and the audio codec unit 140 is executed without passing through the PC 110, it is not possible for the PC 110 to control the flow of video code and audio code. Also, in order for the PC 110 to control the flow of video code and audio code and to store the video code and audio code on the hard disks disposed in the PC 110, a new specialized network control unit 120, and specialized interface software become necessary.
4. Since the specialized network control unit 120 is a specialized network control unit for the AV unit 170, then it cannot use the resources included in the PC 110, such as network control software.
It is possible to conceive a system which does not make use of a separate AV unit 170, wherein a method for the transfer of video code and audio code is achieved with video and audio codec circuits being connected to an AV multiplexer/separator unit composed of software in the PC 110 via the computer bus 114 of the PC 110. However, under such a method, the audio sampling signal in the audio codec unit 140 for the generation of audio code will not be synchronized to the network clock for the ISDN line, so that when the generated video code and audio code are reproduced by the receiver, synchronization slips will occur. In such a case, since especially audio data is reproduced for serial bits, then there will be the problems of audio interference and gaps in the audio signal.